PCF2129A datasheet(18/69 Pages) NXP

18 / 69 page

PCF2129A_2

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Product data sheet

Rev. 02 — 7 May 2010

18 of 69

NXP Semiconductors

PCF2129A

Integrated RTC, TCXO and quartz crystal

The internal power supply (available on pin BBS) is equal to VDD or VBAT. It has to be

assured that there are decoupling capacitors on the pins VDD, VBAT, and BBS.

8.5.2 Battery backup supply

The VBBS voltage on the output pin BBS is equal to the internal power supply, depending

on the selected battery switch-over function mode:

Fig 6.

Battery switch-over circuit, simplified block diagram

001aag061

VCC

Vth(sw)bat

VDD

VBBS

(internal

power supply)

VDD

VDD(int)

VBAT

LOGIC

comparators

logic

switches

VCC

Vth(sw)bat

VBAT

Table 14.

Output pin BBS

Battery switch-over function mode

Conditions

VBBS equals

standard

VDD > VBAT OR VDD > Vth(sw)bat

VDD

VDD < VBAT AND VDD < Vth(sw)bat

VBAT

direct switching

VDD > VBAT

VDD

VDD < VBAT

VBAT

disabled

only VDD available,

VBAT must be put to ground

VDD

Fig 7.

Typical driving capability of VBBS: (VBBS − VDD) with respect to the output load

current IBBS

IBBS (mA)

001aaj327

−400

−600

−200

VBBS − VDD

(mV)

−800

VDD = 4.2 V

VDD = 3 V

VDD = 2 V

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